Glaciers are vast, slow-moving bodies of ice, snow, and rock that originate on land and flow downslope under their own immense weight. These powerful agents of geological change significantly alter Earth’s landscapes. Erosion is the process by which natural forces wear away and transport rock and soil, and glaciers are highly effective at this. Through their movement, glaciers carve, grind, and reshape terrain.
The Dynamic Nature of Glaciers
The ability of glaciers to cause erosion stems from their dynamic movement. Glaciers primarily move through two mechanisms: basal slip and internal deformation. Basal slip occurs when meltwater forms a thin layer at the base of the glacier, allowing the ice mass to slide over the underlying bedrock. This meltwater acts as a lubricant, reducing friction and facilitating the glacier’s movement.
Internal deformation, or creep, involves the gradual distortion of ice crystals within the glacier. Under the immense pressure of its own weight, the ice behaves like a very viscous fluid, causing individual ice crystals to deform and slide past one another. This internal flowing allows the glacier to move even when its base is frozen to the bedrock. Both basal slip and internal deformation enable glaciers to exert tremendous force on the landscape.
Primary Processes of Glacial Erosion
Glaciers reshape the land through two main erosional processes: glacial plucking and glacial abrasion. Glacial plucking, also known as quarrying, involves the glacier lifting and carrying away loosened blocks of bedrock. Meltwater from the glacier seeps into cracks and fractures in the bedrock. As this water freezes, it expands, exerting pressure and widening the cracks, a process known as frost wedging.
When the glacier moves, it “plucks” these weakened rock fragments from the bedrock, incorporating them into its ice mass. Glacial abrasion occurs as rock fragments embedded within the moving ice act like sandpaper. These rock tools, dragged along the underlying bedrock, grind and scrape the surface. This grinding action produces fine rock flour, polishes rock surfaces, and creates parallel grooves called glacial striations, which indicate the direction of ice flow.
Distinctive Landforms from Glacial Erosion
The continuous action of plucking and abrasion sculpts distinctive landforms. One prominent example is the U-shaped valley, characterized by its wide, flat floor and steep, straight sides. Glaciers transform pre-existing V-shaped river valleys into this U-shape by eroding both the valley floor and its sides. This occurs as the glacier moves through the valley.
Cirques are bowl-shaped depressions with steep sides, often found at the head of a glacial valley. They form where snow accumulates and compacts into ice, and the nascent glacier erodes the hollow through plucking and abrasion, deepening the basin. When two adjacent cirques erode towards each other, they can form a sharp, knife-edge ridge called an arête. These narrow ridges often separate two parallel U-shaped valleys. Where three or more cirques erode a mountain peak from different sides, they can create a sharp, pyramid-shaped peak known as a horn, such as the Matterhorn.